The present invention relates to a storage star network in which a maximum propagation delay is assured.
As shown in FIG. 1 the conventional star network is made up of a toll center 1 and a plurality of terminal stations 2A to 2M connected to the toll center 1 through transmission lines 20A to 20M and reception lines 21A to 21M, respectively (Japanese Patent Publication No. 16453/1984).
In the prior art network of FIG. 1 a data packet is transmitted from a terminal station 2A to the toll center 1 through a transmission line 20A. Upon reception of the data packet, the toll center 1 broadcasts the data packet for all the terminal stations 2A to 2M through reception lines 21A to 21M. Each of the terminal stations 2A to 2M judges whether the data packet is addressed to it and receives the data packet if so.
Each of the terminal stations 2A to 2M monitors the the reception lines 21A to 21M, respectively. When a data packet is detected on the reception line the terminal station will not transmit a data packet to the toll center. If a terminal station receives a transmission request, it will begin data packet transmission to toll center 1 after waiting until the data packet in the reception line has been completely transmitted.
However, in the prior art network shown in FIG. 1, if a transmission request is given to a plurality of terminal stations, data packets will collide with each other because the data packets are simultaneously transmitted. The toll center is provided with a device to detect such a collision when two terminal stations or more send out signals at the same time, and informs all the terminal stations of the collision. Upon notification of a collision from the toll center, each of the terminal stations currently transmitting stops sending the data packet and executes a re-transmission algorithm for transmitting the data packet again. This circumstance will be explained with reference to FIG. 1.
First, the station A of the terminal stations sends out a data packet to the toll center through station-A's transmission line 20A. The data packet is illustrated as "station-A transmitted packet" in FIG. 2. Upon reception of this data packet, the toll center broadcasts the data packet to the stations A, B and C. In this example, the respective lengths of the reception lines for the stations A, B and C increase in order and therefore the transmittal of a data packet to the station C from the toll center is delayed most because of differences in propagation time. In FIG. 2, the reception of the data packet by the stations A, B and C are illustrated as "station-A received packet", "station-B received packet" and "station-C received packet", respectively.
Next, assume that station B sends out a data packet and station C begins to send out a data packet before the transmission by station B has been completed. At that time, the packet collision is detected in the toll center at the hatched portion in the drawing. Then the toll center notifies the network that a collision has occurred by either sending out the overlapped signal produced by the collision or sending a collision notification signal. Which ever course of action is taken by the toll center, stations B and C will receive notification of the collision. Immediately upon receiving notification of the collision stations B and C will begin retransmission of their respective data packets. The re-transmitted data packet from station B will reach the toll center before station C's data packet because it takes less time for the signals to travel the shorter distance from the toll center to station B.
If station A, which is even closer to the toll center, subsequently transmits a data packet, a collision between the data packet re-transmitted by station C may occur as illustrated in FIG. 2. The process of notification and retransmission would be repeated and the data packet from station C would be further delayed. This sequence illustrates how this star network can cause considerable delays in transmission of data packets.
Conventionally, in the prior art, data packet collision has been treated in the manner described above.
In the case where the transmission request is given to a plurality of terminal stations, data packets collide with each other because they are simultaneously transmitted. To make it possible that the toll center 1 receives all the data packets without losing the respective data packets upon occurrence of such collision, the inventors have developed a storage star network in which reception memories are provided in the toll center 1 for the purpose of temporarily storing data packets sent from the respective terminal stations, thus to make it possible that the data packets are successively read (polled) as shown in unexamined Japanese Patent Application No. 226570/1986. According to the network, contact time delay occurs between the time of data packet transmission from a terminal station and the time of data packet reception by another terminal station after broadcasting of the data packet from the toll center. The delay is called "propagation delay".
In the storage star network guaranteed to have a maximum propagation delay, the quantity of data packets simultaneously stored in the toll center is limited to guarantee that the maximum propagation delay of the packets sent from the terminal stations is within a predetermined time. Accordingly, if packets from the respective terminal stations are concentrated into the toll center at once, the time required for broadcasting the data packets from all the reception memories of the toll center is within a predetermined time. In other words, data packets sent from the respective stations to the toll center can be broadcast within a predetermined time. The maximum propagation delay of the system depends on the predetermined time.
All the terminal stations in the aforementioned system, however, are not guaranteed to have a maximum propagation delay. Assuming that the maximum propagation delay is 10 msec and that the transmission rate of audio signal is 64 kbps (64 kbit per second), the terminal station dealing with an audio signal must send the packet of about 80 bytes per 10 msec, more particularly, about 100 bytes per 10 msec inclusive of the overhead of the packet. Accordingly, the reception memory in the terminal interface of the toll center must provide at least 100 bytes for the terminal station having one audio channel. On the other hand, for example in the Easanet system (data communication network by Xerox Corp.), the maximum packet length is established to be 1500 bytes. Generally, in such communication between computers, the packet length is required to be so much.
Consequently, the reception memory in the terminal interface of the toll center must provide about 1500 bytes for the datagram terminal station sending the aforementioned packet.
The problems in such a conventional star network as shown in FIGS. 1 and 2 are as follows.
(1) As the circuit becomes crowded, the probability that signals collide with each other increases and thereby causes variations in delay time. Large variations in delay time make the network unsuitable for real time transmissions such as voice communication which stress the real-time relationship between transmission and reception.
(2) Invalidation of signals due to collisions wastes network resources. This waste causes the guaranteed transmission capacity to be far less than the actual physical transmission capacity.
(3) In the case where the system length is so much, a time lag between the time required for transmitting a data packet from a short-distance terminal station to the toll center and the time required for transmitting a data packet from a far-distance terminal station to the toll center becomes large. It is therefore apprehended that packet collision cannot be detected in the toll center. Accordingly, the maximum system length must be limited so that packet collision can be detected.
In a storage star network system guaranteed to have a maximum propagation delay, there is a solution for avoiding data packet collision. That is, assuming that the transmission rate is 10 Mbps in the storage star network system guaranteed to have a maximum propagation delay, 128 audio terminal stations which can be connected to the toll center but no more than 8 Easanet datagram terminal stations. This is because datagram terminal stations and audio terminal stations are all guaranteed to have the same maximum propagation delay. The characteristics of such datagram terminal stations are as follows:
(1) Data transmission can be made without the establishment of connection;
(2) The duty factor is too small;
(3) The probability that minimum length packets and maximum length packets will be sent out is high; and
(4) It is unnecessary to have a guaranteed maximum propagation delay, i.e., guaranteeing a maximum propagation delay for both.
Accordingly, it is not always necessary that datagram terminal stations are treated in the same way as audio terminal stations are treated.